Hi All I was wondering if you guys could please clear up question for me & my friend.
We are both new to the refrigeration trade so are both learning & doubting are selfs some times we are both being told different things by engineers & having a debate on which is correct..

My friend came to & told me he was charging a walk in freezer with 404a but was not getting any of the running pressure that he should have been I've been told by superior for a freezer back pressure of 1bar & head just above 200 psi in not sure what my friend has been told but he was getting no where near these pressure & was told when it's a long pipe run just ignore the pressure & fill to the sight glass basically only use the gauges to check for blockages.. My stand is that you go off sight glass & running pressure not just sight.. I'd have thought it does not matter on the size of the system cause you accomadate for that by adding extra refrigerant..

Please some one clear this up!?

Brian_UK

19-06-2015, 02:31 PM

Yes a sight glass is very useful but like all things until the system is running at the design conditions your readings will not be 'final'.

Also you may find it easier to stick with SI readings, having 1bar suction and 200psi discharge just is nonsensical.

nooney

19-06-2015, 03:44 PM

I wouldn't say it's nonsensical it's just easier 1bar, 16psi -30 it's just the way I have been taught

nooney

19-06-2015, 03:50 PM

I know that if your charging a system & the room is warm you have to account for that temp to drop so your back pressure & head pressure will drop to but when charging you would never just discount the gauges all together & just fill to the sight glass would you.. My friends superior told him on a big system just throw out pressures & just go off the sight glass

Rob White

19-06-2015, 05:34 PM

.

Ask your friend what he thinks the gauges are used for
because he may as well save a lot of money and just use
a plain hose to charge with.

Refrigerants have a pressure / temperature relationship and
dependant on where the system is located and what the product
is inside the coldroom, the running pressures will vary.

the sight glass indicates if the liquid line is full of liquid, this is a
very good indication of whether the system is charged with
refrigerant or not, but the sight glass won't tell you if the system
is running correctly or not.

Your friends superior is a little misinformed and maybe your friend
should pay less attention to him.

There are lots of good publications for learning and there is a good
free learning site at Danfoss.

http://learning.danfoss.com/english

Regards

Rob

.

Tycho

19-06-2015, 06:50 PM

I know that if your charging a system & the room is warm you have to account for that temp to drop so your back pressure & head pressure will drop to but when charging you would never just discount the gauges all together & just fill to the sight glass would you.. My friends superior told him on a big system just throw out pressures & just go off the sight glass

you have to take into account a few things when you charge a system.
-pipe distance between compressor unit and expansion valve
-temperature in the storage room and suction pressure.

if it's a long distance between the compressor unit and the expansion valve, chances are that the guy who installed it installed the sight glass close to the compressor, right after the filter drier.
as a consequence, the sight glass might be clear with liquid, but you have flashing at the TX valve.
The best placement of the sightglass is just before the TX valve, because that is where you need to know if you have enough liquid.

I've sinned myself, because it's easier to place the sightglass close to where you are charging :)

I wouldn't pay any attention to the discharge pressure unless the suction pressure is where it should be.

Whenever I am working on a system with a low charge, or when charging a new system, I always throttle the suction valve to keep the pressure where it's supposed to be during normal operation.
This way I don't have to be bothered by discharge pressure cut-outs, or worrying about the discharge pressure.

Most walk in freezers are only designed to keep the temperature, not to freeze room temperature products, for this reason you can only get the correct charge when the room is at temperature and the suction pressure is where it's supposed to be.

unless the system has a HP receiver, with a level glass where you can eyeball the charge, it's a time consuming job to wait for the room to reach the correct temperature.

Your friends supervisor who said to discount the gauges and just go by the sight glass should get s mack on his noggin :)

If the room temperature is too high, and the suction pressure is too high and you fill until the sight glass is clear, then 10 hours later, when the temperature in the room and the suction pressure has dropped, there will be bubbles in the glass again.
Funny thing about a refrigeration cycle... it all seems to be connected ;)

For me, if the room temperature was too high and I couldn't get the correct suction pressure I would have charged until the sight glass was clear and then a little bit extra. then I would tell the machinist to keep an eye on the sight glass and call me the next day if it started bubbling again when the temperature in the room came down.

it's been quite a few years since I worked on these systems, but back then, if I had a dollar for each time I was called out and had to be a "suction pressure regulator" after the steward on a ship had replenished the provisions, starting by leaving the already frozen goods on land until it was starting to melt, then opening the door to the freezing storage and spending the next three hours loading the nearly thawed provision, and then complaining that the system keeps tripping on discharge pressure and the evaporator keeps icing up, I would have had a couple of hundred dollars now ;)

Bit of a rant and ramble :)

if I had to boil it down to one sentence it would be "take your time, take into consideration all parameters of the system at the time and remember that it will only be correct at design pressure/temperature"

if you pay attention to the system, it will tell you what is wrong :)

Tycho

19-06-2015, 06:54 PM

.

Ask your friend what he thinks the gauges are used for
because he may as well save a lot of money and just use
a plain hose to charge with.

Refrigerants have a pressure / temperature relationship and
dependant on where the system is located and what the product
is inside the coldroom, the running pressures will vary.

the sight glass indicates if the liquid line is full of liquid, this is a
very good indication of whether the system is charged with
refrigerant or not, but the sight glass won't tell you if the system
is running correctly or not.

Your friends superior is a little misinformed and maybe your friend
should pay less attention to him.

There are lots of good publications for learning and there is a good
free learning site at Danfoss.

http://learning.danfoss.com/english

Regards

Rob

.

wise words right there, said it better than me :)
I tend to over explain and a client called me half way through my novel and delayed me :)
Wish I could have explained something that easily, Added reputation :)

RANGER1

20-06-2015, 12:16 AM

I wouldn't say it's nonsensical it's just easier 1bar, 16psi -30 it's just the way I have been taught

nooney don't take this the wrong way, as you asked the question.

1 bar or 14.5psi = -30 deg C

200psi = 31.7 deg C OR 13.7 bar

So what does the above mean to you, do you have any other readings like superheat or room temperature to go with it.

I think Brian is referring to mixing bar with psi, which means NOTHING to a fridge engineer unless they are converted back to temperature.
Hopefully Deg C & Deg F are not mixed up there as well.

If you work on one refrigerant & one type of system, you could be familiar on whats going on, but at the end of the day if you work on something else you would be in trouble without pressure/temp chart.
R404a you could charge all day & possibly still have bubbles in sight glass.

In the good old day's of R12,22 & 502 charging to sight glass was a pretty good indication, but not any more.
Maybe your superior is as old or older than me.

Gary 101 in Forum "Fundamentals" is also very good for learners & you can interact if you want to.

Martin Davies

20-06-2015, 06:34 AM

Hi nooney, I'm fairly new to the trade too (10 years) compared to some of the veterans who have already posted here and so I have some advice based upon my own experience. I personally work in bar and degrees just because that is what I started with and that is what I know. If you start mixing the two up - psi and bar - it can start to get confusing, it's extra things to convert and it's where problems may occur.

Once you are happy with the scale you are using and comfortable reading this accurately on the gauges (gauges are an absolute must by the way) you can begin to convert the pressures into temperatures. I use Danfoss refrigerant slider, find all the apps here....http://refrigerationandairconditioning.danfoss.com/knowledge-center/coolapps/ and find it very useful when charging or just inspecting a system. The rest will come with experience and you will begin to understand how much gas a particular system/pipe-run will take and automatically know when it's not enough or too much. Weighing scales are another must obviously!